1. Field of the Invention
The present invention is related generally to bridges, and in particular to a bridge structure which employs frame structures.
2. Description of the Prior Art
Steel, because of its structural strength, has been used for making bridges. Engineers and designers have been trying to elongate the main span between the main towers because there is a demand for such bridges to span wider obstructs such as straits, wide rivers and bays.
Due to present technology and innovation, only suspension bridges and cable stayed bridges are usually used for long spans. The present type of suspension or cable-stayed bridge is designed with a hanging bridge girder. The deck or box shaped girder of present bridge design is installed without input of any stresses except naturally occurring longitudinal direction tension by its dead load. Especially for long bridges, there is a need to consider that the transverse force, which is generated mainly from winds, is the main force acting against the bridge. Also, there are limitations to the hanging method of constructing the girder for suspension or cable-stayed bridges. Herefore, the deck or box shaped girder has potential strength that is under-utilized because the deck or girder box is not stressed or tensioned prior to the installation.
Regarding the concept of these two bridge designs, for example, suspension bridges, the deck or box shaped girder is simply hung from the main cable. This deck or box shaped girder is the only structure against transverse direction force such as strong winds. This does not protect against the transverse direction force. Of course, the cable contributes some degree of transverse direction. However, cables or girders are basically swayable, especially when transverse force is applied because those are just hung, also, in the case of super-long bridges, after construction of the deck or box shaped girder. It is covered and the road is installed. This is how the load weight, which is mainly from vehicles, is sustained.
In the case of cable-stayed type bridge, the skew cable suspended from the main tower has the role of sustaining the deck or box shaped girder can withstand the horizontal force.
The present concept of design, as explained above, requires that the girder becomes large and heavy in order to resist strong horizontal direction forces, such as strong winds. This large and heavy structure requires stronger and heavier cables and limits the length of its span. This is why the present concept of bridge design has a corresponding problem: how can one build much longer bridges with present cable and steel technology without adding to the size and weight of present methodology.
The main reason is that the character of steel, which originally has very strong tensile strength, is not fully utilized. The present design method of long bridges, especially for suspension and cable-stayed bridges, is as follows:
1. The girder is designed based on the stability and strength against the winds. PA1 2. The strength of the suspension main cable design is based on the girder weight which is noted above. PA1 3. The length of the span is determined based on the limit of tension strength of main cable.
Based on our existing theory or concept of designing long bridges, it is anticipated that the present span length limitation of about 2.about.3 km can now be surpassed.
In addition to the current design problem there is another factor that must be considered.
Usually steel is weaker against compressing forces than against tensile forces. Therefore, the structural steel has to have wider cross section areas to be able to support buckling loads created by a heavier girder structure and protect against strong winds which exert horizontal force on the bridge.
It is clear that if the potential tensile strength of steel is utilized completely against horizontal forces, usually created by strong winds, the minimization of the weight of the girder is possible. When the weight of the girder is diminished, the burden to the main cable is decreased, consequently the central span length can be made much longer. Therefore, in order to design long or super long bridges, the minimization of the girder's weight, through the use of applying potential tensile forces, would be the most important objective.
In addition to the method of how to give the girder pre-tensile force, there is one more aspect which must be considered and that is how to construct those long bridges. According to present construction methods, after hanging each girder, the girders are connected to each other. Because of this, the girders are simply hung to the cable. There is no horizontal direction tension, except the tension of main cable, in the present bridge design system. In fact, as longer bridges are designed, engineers will find that the bridge's span can not be lengthened since the problem just described is not taken into consideration.
As long as the girders are set without horizontal direction tension, the potential strength of steel is not perfectly utilized. A problem occurs because the total strength of the bridge is insufficient. Current bridge design engineering is creating strength of the girder by simply increasing the size of the girder. Therefore, under the present method, to complete the construction of a bridge, more material and expense is required. Consequently the bridge becomes much heavier than an ideal structure which restricts the maximum length of the bridge's span.
Information about the state of the suspension and cable stayed bridge art is provided in the following: U.S. Pat. No. 4,589,156 to Schambeck; U.S. Pat. No. 4,513,465 to Schambeck; U.S. Pat. No. 4,535,498 to Webster; U.S. Pat. No. 4,457,035 to Habegger et al.; U.S. Pat. No. 4,451,950 to Richardson; U.S. Pat. No. 4,253,780 to Lecomte et al.; U.S. Pat. No. 4,208,969 to Baltensperger et al.; U.S. Pat. No. 4,069,765 to Muller; U.S. Pat. No. 3,979,787 to Ahlgren; Japanese Laid Open Applications 4-26002 and 3-80203; and Beard, A. S., "Development of the Tsing Ma Bridge," THE STRUCTURAL ENGINEER, Vol. 71, No. 11, June 1993, pages 192-195.